Mr. Sestak, who has relentlessly pummeled Mr. Specter for not being a “real Democrat,” couldn’t resist taking one more swing at his opponent in a holiday e-mail sent out to supporters last week. The message blamed Mr. Specter, who switched parties earlier this year, for contributing to the economic woes that are spoiling so many celebrations in Pennsylvania this season.

The remark echoed the campaign’s previous criticism that Mr. Sestak, a two-term House member from the town of Media in the western Philadelphia suburbs, is out of his league in the Senate race.

Mr. Sestak’s note, which otherwise extolled good holiday feelings and brought supporters up to date on his campaign, bemoaned Mr. Specter’s failure to take responsibility for supporting the George W. Bush administration’s “failed economic policies.”

“I disagree with the record of voting by our present senator, Arlen Specter, when he supported policies that failed to protect the interests of the working family,” the congressman said in the holiday message. Mr. Sestak went on to list several votes his rival cast that he said either hurt working families or helped Wall Street fat cats, including “repeatedly backing Bush’s deficit-funded tax breaks for multimillionaires, doubling our national debt from $4.5 trillion to over $9 trillion.”

Asked whether the political attack was in keeping with the holiday spirit, Sestak campaign spokesman Jonathon Dworkin said the e-mail was intended as a campaign update.

“We are happy to hear that the senator is keeping up with and thoroughly reading our updates, and hope he and his family enjoyed a happy Hanukkah,” Mr. Dworkin said.

The two Democrats have been battling fiercely since the summer, when Mr. Sestak, a former Navy vice admiral, ignored pressure from President Obama to stay out of the race and challenged Mr. Specter, 79, who switched parties in April after five Senate terms as a Republican.

Mr. Specter traded party allegiances in part to avoid a tough primary challenge from Republican Pat Toomey, a former House member from Pennsylvania with strong conservative backing.

Mr. Sestak, 58, has consistently trailed the incumbent in the polls, but Mr. Specter still faces a close race against Mr. Toomey.

A recent Quinnipiac University poll showed Mr. Specter with a 23 percentage point lead over Mr. Sestak, 53 percent to 30 percent. The same poll showed Mr. Specter and Mr. Toomey tied at 40 percent in a general election matchup.

Peter Brown, assistant director of the Quinnipiac University Polling Institute, said the numbers are troubling for Mr. Specter, even though he remains in a competitive position for the general election.

The survey of more than 1,300 Pennsylvania voters, conducted Dec. 8-14, also found voters split on Mr. Specter’s job-approval rating, 47 percent to 45 percent, and his favorability rating, 43 percent positive to 45 percent negative. Mr. Toomey had a 35 percent to 10 percent favorability rating, but 55 percent of voters say they do not know enough about him to form an opinion.

Meanwhile, Mr. Sestak does not miss an occasion to highlight what he sees as Mr. Specter’s failings.

“However we celebrate the holidays - Christmas, Hanukkah, Kwanzaa, and others - this season always seems to bring loved ones together,” Mr. Sestak said in his holiday e-mail. “So from my family to yours, we wish you joy and happiness heading into the new year.”

After dragging Mr. Specter over the coals for “misguided policies” that he said cost Pennsylvanians jobs and forced some families to lose their homes to foreclosure, Mr. Sestak closed the e-mail by wishing everyone “happy holidays.”